This invention relates to a process for monitoring the temperature of a resistance heating element and, more specifically, relates to a method of monitoring the temperature of a cutting means in an apparatus for forming a sterile connection (sterile docking) between two tubes.
U.S. patent application Ser. No. 267,291 filed on June 4, 198l, now U.S. Pat. No. 4,369,779, discloses an apparatus for forming a sterile connection comprising a cutting means, means adapted to heat said cutting means, a pair of mounting blocks adapted to receive and hold two tubes to be joined, means to provide movement between said blocks and said cutting means to a position such that the cutting means is between said blocks and traversing where the blocks are adapted to receive tubes, means adapted to realign said blocks to a position where two different tube ends are aligned with and facing each other, and means to separate said blocks and said cutting means while urging said blocks together. The application discloses that the process of joining two thermoplastic tubes together transversely of the axis of each tube comprises forming a continuous molten seal between a heated cutting surface and a transverse section of each said tube thereby maintaining a seal between the interior and exterior of said tubes, joining the respective molten ends of said tubes together to form a joint between said tubes while maintaining said seal.
During the use of the sterile docking apparatus according to the aforesaid U.S. application, it has been found that, when the cutting means is a laminated heating element heated by a resistor, various phenomena occasionally occur which result in unsatisfactory performance. These phenomena include (1) when the cutting means is mounted in the sterile docking device, electrical contact resistance may be too high or erratic; (2) the composite cutting means may possess insulating bubbles or may delaminate; (3) convection heat losses may be too great; (4) when a battery is used to supply current to the resistor, the battery current may drop excessively; (5) the current source may malfunction, (6) cutting means may possess shorts or open circuits; (7) or the cutting means may have an initial resistance which is significantly higher or lower than that desired. The foregoing problems may cause the cutting means to fail to heat to the desired temperature within an expected time, may cause the resistor to become too hot within an expected time indicating an unknown change in the correlation between resistor temperature and the temperature of the outside surface of the cutting means, or may cause the cutting means itself to become too hot within an expected time. Improper cutting means temperature can result in a poor mechanical joint and/or a nonsterile joint. A method which minimizes occurrences of either result is quite desirable.
Methods of controlling the temperature of a heating element and methods for shutting down a heating element are known. These methods generally involve a fixed heating element and an initial set-up of the circuit elements to accommodate the specific heating element used, whereas in the aforesaid sterile docking apparatus the heating element (cutting means) is used only once and then replaced. U.S. Pat. No. 3,679,871 issued to Evalds on July 25, 1972 discloses an alternate-current heater using resistance sensing of the heater element to control power to the heater to maintain a predetermined temperature. U.S. Pat. No. 3,789,190 issued to Orosy et al. on Jan. 29, 1974 discloses a control system that uses variable resistance of the heating element to control temperature by alternately applying heating power and sampling resistance several times per second to maintain a predetermined temperature. Other patents disclosing similar systems include U.S. Pat. Nos. 3,869,957 and 3,924,102.
U.S. Pat. No. 4,081,660, issued to Uffer et al. on Mar. 28, 1978, discloses an electric heater shutdown circuit connectable between an electric current source and a heating element in an electric heater and comprising electronic means for shutting off electric current to the heating element when the heating element reaches a predetermined temperature, an electronic means for maintaining the flow of electric current to the heating element shut off after the heating element cools to a temperature below said predetermined temperature, said electronic means for maintaining said electric current shutoff being disabled only upon disconnection of said shutdown circuit from the current source for a predetermined period of time.
U.S. Pat. No. Re. 30,190, reissued on Jan. 15, 1980 to Shaw, discloses a surgical cutting instrument which includes an electrically heated cutting edge and an automatic control system for maintaining the cutting edge at a constant high temperature for sterilizing the blade, cutting tissue, and cauterizing the incised tissue. The blade comprises an electrically heatable element having a plurality of sections which form portions disposed in the region along the tissue-cutting edge and connection means for providing electrical connections to each of said sections for independently supplying electrical power thereto for maintaining the resistance of each of said sections at a substantially constant selected value.
U.S. Pat. No. 4,219,025 issued on Aug. 26, 1980 to Johnson discloses a surgical cutting instrument similar to that disclosed in U.S. Pat. No. Re. 30,190. The patent discloses that the instantaneous temperature of the cutting edge is monitored by measuring the resistance of the heating element or elements and the monitoring signal thus derived provides an inferred value for temperature which may be used to control the power supply to portions or segments of the heating element.
U.S. Pat. No. 4,243,875 issued to Chang on Jan. 6, 1981 discloses a temperature control system for a resistance element which is heated by a power supply connected to it by a switching member, the switching member being controlled by an electronic circuit which is operative to produce alternate on cycles and off cycles whose durations are controlled by separate time constants. The durations of the alternate cycles respectively set the higher and lower temperature limits within which the heating element temperature remains.